I saw Geoffrey A. Landis talking on "Alien Engineering," on the History channel, discussing the possibility of Antigravity.
I don't believe in Aliens, or at least any that are no more then the equivalent of North Americans bothering with monkeys in Africa, but I got to thinking.
Any race of of aliens out there would have to be +1000 to +1,000,000 or more years ahead of us.
It's very likely their concept of a "being" and life would have to be radically different then our current understanding.
Just as we are progressing towards being more machine based, the next evolutionary levels beyond would have to be even more robust, flexible and adaptable.
There would be a strong likelihood that Aliens would not be bound to a specific form or medium.
Their individual entities could be exist as some solid state medium much like our Silicon based RAM but based on quantum computers.
They could then grow themselves into a biological form (flesh) or convert them back to some information based medium as needed. Possibly even transmit them selves as a electromagnetic wave based signals.
Maybe they have even found some way to be in some form of plasma or other non-matter energy based existence that we can't even comprehend at this point.
Point being, they wouldn't be tied to a flesh and blood body, but exist more as information and software then animal.
So just like our software, that could be in the form of RAM, Hard disk, CD, USB Flash drive, or sent over the internet or radio signals their whole beings could be just as flexible.
They should be able to assume forms optimized for 0G , 1G or even 9000+G and transform themselves as needed for the given situation as easily as we would don a coat and gloves on a cold day.
Same with vehicle's and modes of transportation. They could easily fabricate physical aircraft or plasma based crafts on demands, when needed. I am also sure that they would have to have solved their energy requirements also, so they would have unimaginable large amounts energy available for propulsion. (Power to weight ratio's approaching infinity)
In any case, it is very unlikely that they would require the use of an Anti Gravity device or even need to violate any of the physics that we currently understand to accomplish many of the feats that we credit UFO's with today.